The present invention relates to a yarn sensor for optically scanning a yarn moving in its longitudinal direction in a measurement gap, particularly for detecting extraneous fibers.
European Patent Disclosure EP 1 018 645 B1 describes a yarn sensor for detecting extraneous material in the yarn, in which a white-light light-emitting diode serves as the light. source. A white-light light-emitting diode makes it possible to embody the yarn sensor compactly. The broad color spectrum of the white-light LED (The abbreviation “LED” is commonplace for light-emitting diodes) prevents the occurrence of an unequal color sensitivity of the yarn sensor, or the failure to detect some colors. The detected measured values are examined for characteristics that can indicate that extraneous substances, such as extraneous fibers, are present in the yarn.
European Patent Disclosure EP 0 761 585 A1 describes a generic type of yarn sensor that can likewise serve not only to determine the yarn diameter, but also to detect extraneous material in the yarn, such as extraneous fibers or contaminants. In yarn sensors, the deposition of dirt, such as dust and fiber particles, on surfaces of the measurement gap that are unprotected against becoming soiled, is unavoidable. It is true that a certain self-cleaning effect occurs in the measurement gap, brought about by the yarn moving through the measurement gap. However, this known self-cleaning effect cannot prevent surfaces in the measurement gap, on which light from the light source shines, from becoming at least temporarily soiled. For instance, if the protective plate between the yarn and the receiver for directly transmitted light becomes soiled, not only is the quantity of light arriving at this receiver reduced, but the dirt particles also reflect the arriving light projected by the light source. The light reflected by the yarn, because of the small surface area of the yarn, represents a relatively small signal source. The yarn signal converted into current varies in the nanoampere range. Compared to the small irradiated surface of the yarn, the relatively large, dirty surface of the measurement gap, because of its length, represents a not inconsiderable source of reflection signals. The interfering radiation, which adulterates the result of the measurement, is also called a parasitic signal. Because of the low intensity of the yarn signal, a high amplification of the signal converted from the incident light at the yarn takes place, but high amplification of the parasitic signals occurs as well. This leads to an impermissibly small useful signal, in proportion to the total signal.
Both the yarn sensor of European Patent Publication EP 0 018 645 B1 and the yarn sensor of European Patent Publication EP 761 585 A1 are incapable of overcoming this disadvantage.